The present invention relates to an isolated DC-DC converter and more particularly to an isolated DC-DC converter with an active clamp circuit.
Referring to FIG. 8 showing the circuit diagram of an isolated DC-DC converter with an active clamp circuit according to background art, the main switch Q1 and the auxiliary switch Q2 are used in the circuit for the primary winding of the transformer 50. The auxiliary switch Q2 is connected in series to a clamp capacitor C thereby to form an active clamp circuit. The active clamp circuit is connected in parallel to the primary winding of the transformer 50. The main switch Q1 and the auxiliary switch Q2 are turned ON and OFF alternately by a control circuit (not shown). A driving signal for the auxiliary switch Q2 is provided by reversing the driving signal for the main switch Q1.
In starting the DC-DC converter with an active clamp circuit, soft start is implemented for preventing bias excitation of the transformer 50.
FIG. 9 shows the timing chart of operation of the isolated DC-DC converter during its soft start period. As shown in the drawing, the duty cycle of the auxiliary switch Q2 is larger than that of the main switch Q1. The term of the duty cycle means the ratio of the time period during which the switch is ON to the sum of the time period during which the switch is ON and the time period during which the switch is OFF. The product Vc·T2, where Vc represents the voltage applied to the transformer 50 and T2 represents the time period during which the auxiliary switch Q2 is ON, is larger than the product V1·T1, where V1 represents the voltage applied to the transformer 50 and T1 represents the time period during which the main switch Q1 is ON. Therefore, depending on the magnitude of electric charge with which the clamp capacitor C is charged at the initiation of soft start of the converter, or on the voltage of the clamp capacitor C, bias excitation is induced and the transformer 50 may be magnetically saturated if the bias excitation is continued. The magnetic saturation of the transformer 50 causes a rapid decrease of its inductance thereby to increase exciting current rapidly, so that excess current flowing through the auxiliary switch Q2 may cause damage to the auxiliary switch Q2. Although the use of a switching element having a capacity to withstand a large voltage and a large current as the auxiliary switch Q2 enables prevention of the damage of the auxiliary switch Q2, it increases the cost of the converter.
Japanese Unexamined Patent Application Publication No. 2006-191741 discloses an isolated DC-DC converter with an active clamp circuit. In this publication, when the control circuit controlling the operation of the main switch and the auxiliary switch is started, power is supplied to the control circuit from the clamp capacitor. Electric charge with which the clamp capacitor is charged in starting the control circuit is transferred to a capacitor forming a power supply of the control circuit, thereby discharging the electric charge stored in the clamp capacitor. Such a converter needs an additional circuit, so that the converter is complicated and its manufacturing cost is increased.
The present invention, which has been made in light of the above problems, is directed to an isolated DC-DC converter with an active clamp circuit, according to which the transformer is prevented from being magnetically saturated after initiating soft start of the isolated DC-DC converter and the switching element having a capacity to withstand a large voltage and a large current as an auxiliary switch of the active clamp circuit is dispensed with.